Skin packaging is essentially a vacuum forming process. In a typical process, a sheet of thermoplastic film is placed in a frame, and below the frame is a vacuum plate upon which a piece of backing board is placed. The product to be skin packaged is positioned on top of the backing board and heat is applied to the thermoplastic film in the frame. When the film has been heated to become sufficiently soft, the frame is lowered and the plastic sheet drapes itself over the product. As this happens, a partial vacuum is created through the vacuum plate and the air underneath the plastic film is withdrawn through the backing board. The air pressure differential between the top and the bottom of the plastic sheet causes the sheet to be tightly pressed around the product. The film may be coated with an adhesive or the backing board may be so coated. Where the two contact each other, a strong bond is formed resulting in a package in which the product is tightly held to the backing board for safe shipping and for subsequent rack display in retail stores.
Vacuum skin packaging differs from the above described skin packaging process in that both the thermoplastic film and the backing board are impervious to gases and the resulting package can be evacuated and hermetically sealed, if desired. The same end result is sought, i.e. the product is to be tightly held by the transparent film to the backing board. The conventional method employs a backing board which is porous or which is perforated so that the vacuum may be drawn directly through the backing board. The vacuum skin packaging processes generally employs a vacuum chamber with an open top. The product on an impervious backing board is placed on a platform within the vacuum chamber. The top of the chamber is then covered by a sheet of film which is clamped tightly against the chamber to form a vacuum type closure. The chamber is evacuated while the film is heated to forming and softening temperatures. The platform can then be raised to drive the product into the softened film and air pressure can be used above the film to force it tightly around the product. This type of process is disclosed in French Pat. No. 1,258,357 issued to Alain G. Bresson on Mar. 6, 1961.
A refinement to the process described in the Bresson French Pat. is disclosed in French Pat. No. 1,286,018 issued on Jan. 22, 1962 to Laroch Freres, Limited. In the Laroch Freres process, after the chamber has been evacuated and the product driven into the heat softened film, the vacuum is released and ambient air is permitted to enter the chamber so that the thermoplastic film molds more or less on the product since there is a vacuum on the product side of the film and ambient air pressure on the other side of the film.
In Australian Pat. No. 245,774 issued to Colbro Proprietary Limited and Cole and Son Proprietary Limited on July 16, 1963, a vacuum skin packaging process is described in which an article to be packaged is inserted within the lower half of a vacuum chamber on a backing board, a thermoplastic film is placed over the open face of the lower half of the chamber, the chamber is closed and both halves are brought to essentially the same state of vacuum, the film heated and softened, and then atmospheric air is introduced into the upper half of the chamber so that it alone forces the thermoplastic film 6 down around the product and against the backing board.
Still another variation which can be found in the prior art, is that disclosed in U.S. Pat. No. 3,491,504 issued to W. E. Young et al. on Jan. 27, 1970. The Young patent discloses a process in which the softened film can be physically moved down over a stationary product and, in combination with air pressure, the softened thermoplastic film will be molded onto the product.
In all of the above described prior art processes, the thermoplastic film is stretched across the open face of a vacuum chamber. The product is then either driven up into the film, the film pulled down over the product, or air pressure is used to move the film. Having to physically move the film or the product slows down the packaging process and, in addition, requires that the product be strong enough to withstand the force of contacting the film. Furthermore, when the film is stretched flat and straight across the vacuum chamber, excess film is required and wrinkling of the film may occur due to unnecessary movement of the film. These disadvantages are overcome and many advantages are provided by the invention described hereinbelow.